The present invention relates to multi-objective energy management methods for micro-grids.
A micro-grid is the integration of loads, energy resources, and storage devices. From the operating point of view, a micro-grid is counted as one independent entity which is able to work either in grid-tied or islanded mode. Micro-grid's energy resources can include utility connection, micro-gas turbines and renewable generations such as fuel cells, wind turbines, and solar panels. It is expected and desirable that a considerable amount of demand for each micro-grid is supplied by its own renewable generations. On the other hand, the intermittent nature of most distributed generations (DGs) such as wind and photovoltaic (PV) introduces a significant uncertainty in the operation of a micro-grid. This makes the conventional unit commitment more erroneous and unreliable. Therefore, a real-time management framework as a supervisory control is an absolute necessary procedure within a micro-grid similar to the various regulatory actions in conventional power systems. The first objective for this management system is real-time dispatching of energy generations in a way that minimizes the operational cost while it guarantees the balance between supply and demand at the presence of unpredictable variations of DGs.
In order to relax the issue of sudden unpredicted unbalances between supply and demand, energy storage devices are normally utilized. Among various types of storage devices, batteries are most favorable option and also the most expensive component of micro-grid. In grid-tied micro-grids, any shortage in the supply side (power outputs from DGs and the scheduled power from the grid) should be met whether by the battery or by purchasing extra power from the grid or a combination of both. At the first glance, it might be preferred to use battery first since it is charge free. But irregular usage pattern of batteries shortens their life span and may cause a replacement cost for batteries. Three parameters affect batteries' life: 1—Depth of discharge (DoD) 2—Discharge power and 3—temperature.
It is expected and desirable that a considerable amount of demand for each micro-grid is supplied by its own local generations. The intermittent nature of most distributed generations (DGs) introduces a significant uncertainty in the operation of a micro-grid. Having more comprehensive forecasting about the renewable generations, we are able to manage the micro-grid more efficient. Hence, a long term predictive controller seems to be necessary to make the optimal decisions based on long term forecasted profiles of renewables and load. On the other hand, we need a real-time manager. It means the controller should be able to guarantee the second-by-second balance between supply and demand and deal with all fluctuations in the system. Hence, as the first problem, a management system is required to optimize both long term predictive control objective and real-time control objective simultaneously.
In order to relax the issue of sudden unpredicted unbalances between supply and demand, battery is normally utilized which is also the most expensive component of micro-grid. On the other hand, irregular usage pattern of battery shortens its life span and may cause a replacement cost for battery. Based on this idea, in micro-grids operation, it should be tried to utilize battery's power in a way that maximizes their lifetime. Therefore, maximizing the battery lifetime is an important objective that should be considered besides the energy cost minimization objective in the operation of micro-grids. Therefore, as the second problem, a multi-objective management system is needed to optimize the battery lifetime and energy cost at the same time.
Previous attempts were based on passive control of energy storage units. An example is schedule-based control in which a storage unit charges and discharges at certain times during the day. There are also some other researches which attempt to optimize the operation of micro-grid by solving an optimization problem. But they just consider and minimize the energy cost of operation; thus they cannot guarantee an optimal operation of the storage unit.